Photon-nucleus energy dependence of coherent J/$\psi$ cross session in ultraperipheral PbPb collisions at $5.02~\mathrm{TeV}$ with CMS

To understand the structure and dynamics inside an atomic nucleus at its most fundamental level is a subject at the heart of research on the strong interaction and theory of quantum chromodynamics (QCD). Gluons, the strong interaction carrier, are found to be the dominant constituents inside a nucleus when it is probed at ultra-high energies. Quasi-real photons exchanged in the interactions of highly relativistic heavy ions are powerful and clean probes of the gluonic structures of nuclei and/or nucleons. We report the first measurement of the nuclear gluonic structure probed by high-energy photons via coherent J/$\psi$ photoproduction as a function of photon-nucleon center-of-mass energy ($W^{Pb}_{\gamma N}$) up to $W^{Pb}_{\gamma N} \approx 400$ GeV. Results are obtained with ultraperipheral PbPb collision data collected by the CMS experiment at the LHC during the 2018 PbPb run, corresponding to an integrated luminosity of $1.52~\mathrm{nb}^{-1}$. At high $W^{Pb}_{\gamma N}$ the coherent J/$\psi$ cross section is observed to rapidly approach an approximate plateau over $40\leq W^{Pb}_{\gamma N}\leq 400$ GeV. This observation may provide the first experimental evidence of the black disk limit reached in high energy photons scattering off heavy nuclei, or indicate a gluon density saturation inside the nucleus that is greater than predictions from state-of-the-art QCD theoretical models. The findings open a new domain of QCD studies in the high gluonic density limit and motivate the development of new theoretical models.